The membranes of secretory granules are inserted into the plasma membrane following the release of secretory products by exocytosis. A monoclonal antibody, 5G10, has been raised which recognizes the membranes of secretory and Golgi-derived vesicles. Fusion of secretory granules with the plasma membrane can be followed by the appearance of the 5G10 antigen on the cell surface. The process is fast and maximal levels of the 5G10 antigen, a 80 kdalton glycoprotein component of the membrane of secretory granules, are measured in the cell surface by five minutes after stimulation of secretion in rat basophilic leukemia cells. The 5G10 antigen is rapidly and efficiently internalized after insertion in the cell surface by endocytosis. The half-life of this internalization is about 10 minutes. The endocytosis of the membrane components of secretory granules allows the cell to maintain a constant chemical composition of the plasma membrane. Endocytosis of the 5G10 antigen inserted into the membrane is initiated via coated pits. The internalized antigen after transiting through the Golgi is found again in secretory granules. The antigen remains there until specific secretion is stimulated.